For carbides of metals belonging to Group 4, Group 5 and Group 6 elements (Group IVa, Group Va and Group VIa elements in the old group number, respectively) on the periodic table, have been used as typical hard materials in wide application. For example, a cemented carbide predominantly composed of tungsten carbide is used as material for cutting tools and abrasion-resistant tools, titanium carbide is used as main material for cermet cutting tools, tantalum carbide and niobium carbide are used as an initial material of a cemented carbide used for watch cases and the like, and chromium carbide is used as material for hard coatings by sprayed coating method. With respect to these materials, an enhancement in their strength and hardness and an improvement in wear resistance have been desired.
Further, in recent years, a cemented carbide and a sintered tungsten carbide are used for molds for aspheric lenses, and requirement for a reduction of surface roughness is increasing reflecting the requirements for smaller, lighter weight and more precise optical equipment.
In order to respond to these requirements, it is essential to make an initial material powder finer and purer, and the needs of fine metal carbide particles having a small average particle diameter and a low concentration of impurities have increased.
Conventional methods used for manufacturing metal carbide powder include a method in which a metal and a carbon powder is mixed and a carbonization reaction is performed by heat-treating the mixture at an high temperature, a method in which metal oxide powders are reduced and mixed with carbon powders and heat-treated at an high temperature in a carburizing atmosphere in order to perform a carbonization reaction, and the like. However, a purification process having multiple steps for obtaining a metal or a metal oxide used as an initial material in these methods from initial mineral ores is extremely complicated, and this process constitutes a major cause of higher costs.
Since metallic impurities such as iron cause a decrease in the purity of the metal carbide and production of an abnormal structure when a sintered body is prepared, there is a problem of how to reduce the metallic impurities in manufacturing the metal carbide. However, in a conventional method, metallic impurities such as iron contamination resulting from a stainless boat which is used in reduction of a metal oxide, pyrolysis of a metal precursor, or carbonization of a metal are inevitable. Further, a carbon powder used as a carbon source often contains iron as impurities, and this is one of the causes of the contamination of a metal carbide with iron. Moreover, since most of the metal carbides manufactured by these methods have a large particle diameter of 1 μm or more, the metal carbides are pulverized to fine powders by milling for a long time with a ball mill and the like. Therefore, contamination with impurities such as iron resulting from a stainless steel pot or a cemented carbide ball media, and oxidation of a carbide powder are inevitable, and it is difficult to manufacture a metal carbide powder having a high purity of 99.9% or more.
Accordingly, a technique for manufacturing a high-purity nano powder by a build-up process without a heat-treating step and a pulverizing step causing the contamination with iron is desired.
For example, Patent Document 1 proposes a method of manufacturing a metal carbide comprising steps of: preparing a solution containing ammonium paratungstate and an organic acid such as glycine; drying the solution by spray drying method to make a solid precursor; calcining the solid in an inert environment to form a partially carburized metal; and heating the metal in a carburizing atmosphere to complete the carburization.
Patent Document 2 proposes a method of manufacturing a high-purity fine tungsten carbide powder comprising steps of drying a slurry formed by mixing a carbon powder in an aqueous solution of ammonium metatungstate; reducing the resulting mixture in a nitrogen atmosphere to prepare an intermediate product containing W, W2C, and WC;, and mixing a carbon powder to the reduction product in such an amount that a W component is carbonized to WC composed of W and C in an atomic ratio of 1:1; and carbonizing the reduction product in a hydrogen atmosphere to manufacture a fine tungsten carbide powder having an average particle diameter of 0.5 μm or less.
Patent Document 3 proposes a method of manufacturing an ultrafine WC/TiC/Co composite powder in which the ultrafine WC/TiC/Co composite powder can be prepared at a low reaction temperature by a simple process comprising steps of: preparing an initial powder from a water-soluble salt containing W, Ti and Co by a spray drying method; removing moisture and a salt component from the powder by heating to convert the powder to an oxide powder; then mixing the oxide powder with carbon; and reducing and carburizing the resulting mixture in an atmosphere of a reducing gas or a nonoxidizing gas.
Patent Document 4 proposes a carbide powder having a maximum particle diameter of 150 nm or less which is obtained by subjecting a precursor solution prepared by dissolving a metal alkoxide (W, Ta, Nb, Cr, Si) and an organic compound having a functional group capable of coordinating to the metal alkoxide in an organic solvent to drying and a heat treatment in a nonoxidizing atmosphere, and a method of manufacturing the same.
Patent Document 5 proposes a tungsten carbide powder having a nano particle size and a method of manufacturing the same, comprising the first heat treatment step of heating a mixture of ultrafine tungsten oxide (WO3 or WO2.90) and a carbon powder to 1050 to 1200° C. in nitrogen to reduce and carbonize the mixture to an intermediate product in which W, W2C, and WC coexist, and the second heat treatment step of heating the intermediate product, or the intermediate product subjected to pulverization and mixing to 900 to 1300° C. in H2 to carbonize the intermediate product to a tungsten carbide powder, as a method of manufacturing a tungsten carbide powder in which a ratio of a total carbon amount is 6.13±0.30 wt %, a free carbon amount is 0.3 wt % or less, an oxygen amount is 0.7 wt % by mass or less, an iron amount is 200 ppm or less, and an average particle diameter is 100 nm or less.
Patent Document 6 proposes a method in which a tungsten compound such as ammonium metatungstate or tungstic acid is used as a precursor composition, the precursor composition is reacted with hydrogen as a reducing agent and a carbon-based gas such as CO as a carbon source in a flow of these gases, and the reactant is caused to undergo a reduction and carbonization reaction in a furnace to manufacture a WC powder.